[ios, macos] remove references to [MGLOfflineStorage sharedOfflineStorage] from internal methods.

[julianrex]

iOS/macOS versions: 4.7.0

We should consider removing the explicit dependency on [MGLOfflineStorage sharedOfflineStorage] from MGLOfflinePack, MGLMapSnapshotter and MGLRendererConfiguration.

This would pave the way to support multiple offline storage objects which would be especially useful during (parallel) testing.

[1ec5]

These classes refer to the shared object via the class method to avoid maintaining a backreference to the file source. Most of the actual offline pack functionality lives in MGLOfflineStorage rather than MGLOfflinePack. This architecture is somewhat dictated by mbgl::OfflineManager, which isn’t quite as object-oriented as what we expose publicly on iOS and macOS. All three classes depend on the file source, which exists in mbgl as a singleton.

[stale]

This issue has been automatically detected as stale because it has not had recent activity and will be archived. Thank you for your contributions.

[stale]

This issue has been automatically detected as stale because it has not had recent activity and will be archived. Thank you for your contributions.